The present invention relates to an image forming method.
In color toner images in which a desired color is formed by laminating toner layers having three primary colors (color layering), with the view of improving color reproduction properties, it is required that color matching be conducted by smoothing the surface (interface) of the toner layer surface of each color. In order to smooth the interlayer of said toner layers, it is advantageous to use a toner having a low melt viscosity during fixing. On the other hand, widely employed, as a system to fix color toner images, is a heating roller fixing system utilizing a fixing unit provided with a heating roller as well as with a pressure roller.
However, when color toner images, comprised of toners having a low melt viscosity, are fixed utilizing said heating roller fixing system, and subsequently the fixed image is peeled off from said heating roller, so-called offset phenomena tend to occur. In order to minimize said offset phenomena, a relatively large amount of silicone oil is applied to the surface of said heating roller so that adhesion between said heating roller and said toner is reduced.
However, in such a system, problems occur in which silicone oil which is applied to the surface of the heating roller is soaked into the transfer paper sheets (an image forming support) to result in degradation of writability of said transfer sheet, and in addition, said silicone oil adheres to the leading edge of the transfer sheet which is brought into contact with said heating roller. Further, when PET sheets, for overhead projection use, are employed as the image forming support, problems also occur in which said silicone oil remains on the surface of said PET sheet to result in stickiness.
When color images are fixed employing the heating roller system described above, no means has been known which sufficiently exhibits both color reproduction properties (smoothness of each color toner layer) and offsetting resistance (releasability in a fixing unit in which no silicone oil is supplied or only a very small amount of silicone oil is supplied).
Multicolor image forming methods include a method in which an electrostatic latent image corresponding to a color is formed on a latent image bearing body, and a multicolor image is formed by repeating the development and transfer process for each color. Said method, however, requires that said electrostatic latent image bearing support is subjected to repetition of development and transfer processes under the frequency of developing each color. Therefore, the rate of image formation is low compared to that of a single color image formation. Thus, when full-color images are formed, said rate is not more than xc2xc of the single color rate.
Accordingly, when said method is utilized, it is difficult to increase the rate of full-color image formation.
On the other hand, methods, in which even full-color images are formed at high speed, include a method, generally called a tandem method, in which an electrostatic latent image bearing body corresponding to each color is provided, and an image formed on each body is transferred so as to form a full-color image.
In said method, since a latent image forming process, a development process, and a transfer process are formed for each color, the rate of image formation is the same as that of a single color formation. Thus, said method exhibits an advantage which makes it possible to achieve full-color image formation at high speed.
However, in said method, since each color image is formed utilizing an individual electrostatic latent image bearing body, it is required to stabilize the developed amount of each color to control the color balance. Further, since an image is formed by transferring a toner image formed on each electrostatic latent image bearing body onto an image support and subsequently by fixing the transferred image, problems occur in which when adhesion of any of color images onto said image support is different, image stability during fixing deteriorates.
The electrostatic latent image bearing body as previously described is commonly an electrophotographic photoreceptor, which is hereinafter referred occasionally to simply as a photoreceptor.
On the other hand, preferably employed as a fixing method is a so-called heating roll method due to the simplicity, and the like, of such a unit. In this fixing method, since the fixing roller is brought into contact with a toner, releasing agents are preferably incorporated into said toner to assure sufficient releasability.
However, in a toner which is prepared employing a conventional pulverization method, materials dispersed into toner particles exist non-uniformly on a broken surface. Particularly, materials such as releasing agents, which melt quickly in the narrow temperature range, tend to be concentrated on the surface so that surface properties among toner particles tend to be non-uniform. Due to that, it becomes difficult to stabilize the developed amount as well as to assure uniform adhesion among the toner particles of each color. As a result, problems occur in which fluctuation tends to result and color reproduction properties, required for quality color images, are degraded.
On the other hand, a so-called polymerization toner has been known which is prepared utilizing a polymerization method. Of said polymerization toners, a toner prepared by utilizing a suspension polymerization method is comprised of spherical toner particles with uniform surface properties. Thus, it is supposed that uniformity among toner particles is enhanced. However, the particle shape tends to become spherical and adhesion onto an electrostatic latent image bearing body as well as onto an image support is enhanced. As a result, problems tend to occur in which transferability is degraded and image repellency during fixing occurs.
As a result, by employing the image forming method comprised of said tandem method, at present, it is difficult to obtain consistent image quality over an extended period of time.
Known as a color image forming method has been a method utilizing a so-called intermediate transfer system in which a latent image formed on a latent image bearing body is developed employing a toner; instead of transferring the resulting toner image directly onto an image forming support, it is temporarily transferred onto an intermediate transfer body, and subsequently retransferred onto said image forming support.
In said intermediate transfer method, by transferring a toner image formed on a latent image bearing body onto an intermediate transfer body several times (for example, 4 times), each color toner layer is laminated on said intermediate transfer body (color layering). In this case, when the last color toner image is transferred onto the intermediate transfer body, the charge amount of the toner, constituting the color toner image which has been first transferred onto said intermediate transfer body, decreases occasionally. In order to minimize the decrease in the charge amount of the toner image in the lower layer, it is required to employ a toner with stable chargeability.
On the other hand, widely employed as a means to fix toner images formed on an image forming support, is a heating roller system, employing a fixing unit provided with a heating roller as well as with a pressure roller.
In said system, a toner is brought into contact with the surface of said heating roller. As a result, so-called offset phenomena tend to occur. Herein, in order to minimize said offset phenomena, a method is employed in which a relatively large amount of silicone oil is applied to the surface of said heating roller so as to provide releasability.
However, when a relatively large amount of silicone oil is employed, problems occur in which silicone oil adheres onto an image forming support and the mechanism of the fixing unit itself becomes more complex. Accordingly, it is desirous that releasability be provided in the toner itself so that the coating amount of silicone oil required for the fixing unit itself can be reduced, or silicone oil is not at all necessary.
In the intermediate transfer system as previously described, when a toner produced employing a pulverization method is utilized, materials dispersed into toner raw materials exist non-uniformly on a broken surface. As a result, it becomes difficult to make the surface properties among toner particle uniform. As a result, problems occur in which transferability tends to fluctuate, and accordingly, color reproduction properties required for quality color images are degraded.
Further, when a toner, produced employing a suspension polymerization, is utilized, an advantage is obtained in which stable transferability is obtained due to uniform surface properties among toner particles.
However, in said suspension polymerization, it is extremely difficult to place releasing agents on the surface of toner particles. As a result, the fixability (particularly, releasability) of toner particles is limited to employed resins, that is, approaches the properties of said resins. Subsequently, problems occur in which offset phenomena occur due to insufficient releasability of toner particles, and image staining results due to the resulting offset phenomena.
As a means to overcome such problems, for example, Japanese Patent Publication Open to Public Inspection No. 10-97098 proposes a suspension polymerization toner which incorporates a large amount of releasing agents in the interior of the toner particles.
However, in such a suspension polymerization toner, since a large amount of releasing agent is incorporated in the interior of toner particles, problems occur in which, due to a decrease in light transmittance caused by the domain of releasing agents, the smoothness of the color image after the fixing process is degraded, resulting in color contamination, and the color reproduction range, which is obtained employing a plurality of colors, is narrowed.
When color images are formed employing the intermediate transfer system as previously described, means have not yet been known which simultaneously result in sufficient color reproduction properties (smoothness of each color toner layer) and fixability (releasability). As a result, it has been difficult to form consistently quality color images.
An object of the present invention is to provide a toner capable of forming color images which exhibit excellent fixability and offsetting resistance, as well as excellent color reproduction, even when employed in an image forming method comprising a fixed color image forming process employing a fixing unit to which no silicone oil, or a very small amount of silicone oil is supplied.
Another object of the present invention is to provide an image forming method capable of forming no offset phenomena, increasing the fixing ratio, and exhibiting excellent color reproduction properties, while employing an image forming method comprising a fixed color image forming process, employing a fixing unit to which no silicone oil, or a very small amount of silicone oil is supplied.
The other object of the present invention is to provide a color image forming method and an image forming apparatus which results in excellent and consistent color reproduction properties over an extended period of time, and an electrostatic latent image developing toner employed in them.
An object of the present invention is to provide a toner capable of forming color images with excellent offsetting resistance as well as excellent color reproduction properties in the wide range for an extended period of time, in the case of forming color images employing an intermediate transfer system, even when said toner is employed in an image forming method comprising a fixed color image forming process, employing a fixing unit to which no silicone oil or only a very small silicone oil is supplied.
Another object of the present invention is to provide an image forming method capable of forming no offset phenomena as well as forming color images with excellent color reproduction properties for an extended period of time in the case of forming color images, even when there is included a fixed color forming process, employing a fixing unit to which no silicone oil or only a very small silicone oil is supplied.
Said fixing unit is employed in an image forming method in which a toner image formed on said photoreceptor is transferred onto an image forming support, and the transferred toner image is fixed employing a fixing unit. Said fixing unit is comprised of a heating roller and a pressure roller which is brought into contact with said heating roller. Said heating roller is constituted in such a manner that the surface of a cylinder having an interior diameter of from 10 to 70 mm and a wall thickness of from 0.1 to 2 mm, comprised of a metal or a metal alloy, is covered with an elastic martial having an Asker hardness C of less than 70 degrees at a thickness of from 0.1 to 30 mm, and a heating member is incorporated in its interior. Said pressure roller is constituted in such a manner that the surface of a metal cylinder is covered with an elastic material having an Asker hardness C of less than 80 degrees at a thickness of from 0.1 to 30 mm.
The supplied amount of silicone oil to said heating roller, comprised of said fixing unit, is preferably not more than 2 mg/A4 sized sheet of paper.
In a toner which is employed in an image forming process which is comprised of a process in which a latent image is formed on a latent image bearing body, a process in which said latent image is developed employing a developer comprising said toner, a process in which a toner image formed on said latent image bearing body is transferred onto an intermediate transfer body, a process in which the toner image formed on said intermediate transfer body is transferred onto an image forming support, and a process in which the toner image transferred onto said image forming support is fixed employing a fixing unit which utilizes a heating roller fixing system, a toner of the present invention is obtained by salting out/fusing resinous particles comprising at least a binding resin containing a releasing agents, a colorant, and a releasing agent, and colorant particles.
xe2x80x9cSalting-out/fusionxe2x80x9d, as described in the present invention, refers to an operation in which salting-out (aggregation of particles) and fusion (disappearance of the interface between particles) occur simultaneously, or salting-out and fusion are allowed to occur simultaneously. In order to simultaneously carry out salting-out and fusion, it is required that particles (resinous particles and colorant particles) are aggregated at a temperature which is equal to or higher than the glass transition temperature (Tg) of the resin constituting said particles.
The inventors of the present invention have investigated the composition of toners which are required to effectively minimize offset phenomena during fixing. As a result, it was discovered that it was possible to effectively minimize the offset phenomena by employing a coalesced type toner which was obtained by salting out/fusing resinous particles comprising releasing agents, together with colorant particles in a water based medium.
Generally, the toner, into which releasing agents are incorporated, results in a decrease in light transmittance due to the domain of said releasing agents. Therefore, it has been considered that it is impossible to use releasing agents in a color image forming toner. However, in the present invention, it is possible to minimize the color contamination of color images, as well as to enhance transparency, by incorporating releasing agents which require said domain structure and employing a method which salts out/fuses said resinous particles. As a result, it has become possible to improve color reproduction properties of fixed color images.
Further, the following has been discovered. The surface of a heating roller as well as a pressure roller in a fixing unit is formed employing an elastic material so that the nip width is widened. As a result, it is possible to maintain the fused state of a toner during fixing and to readily form a toner layer on the smoothed interface so that it is possible to enhance the color reproduction properties.
However, when such a constitution of a fixing unit is only employed, the nip width during fixing is widened, while a peeling angle in the fix peeling section increases so as to result in a state in which offset phenomena tend to occur.
Therefore, it is possible to exhibit the effects of the present invention by combining said fixing unit with a specified coalesced toner (the toner of the present invention), instead of employing only said fixing unit.
(1) Since the toner of the present invention is a coalesced type toner prepared employing a salting-out/fusion method, the surface properties among toner particles is uniform. As a result, it is possible to achieve excellent fixability
(2) Since the toner of the present invention is comprised of toner particles, having releasing agents (which have been incorporated into resinous particles) on its surface, it is possible to achieve excellent offsetting resistance.
(3) Since the toner of the present invention is obtained by coalescing resinous particles containing releasing agents, employing said salting-out fusion method, color contamination problems which result in the simple addition of said releasing agents do not occur, and high light transmittance is achieved. As a result, it is possible to form fixed color images which exhibit excellent color reproduction properties.
(4) Since each of the heating roller and the pressure roller, which constitute a fixing unit, is covered with an elastic material having low hardness, the resulting nip width is widened, making it possible to form a smoothed toner layer at the interface. As a result, it is possible to improve the color reproduction as well as to enlarge the color reproduction range.
(5) As previously described, by combining the specified coalesced type toner with the specified fixing unit, it is possible to simultaneously achieve excellent offsetting resistance as well as excellent color reproduction of fixed images.